Module 2 : Global Positioning System
  Lecture 13 : GPS Measurements Techniques II
Table 12.3 summarizes requirements for different types of GPS surveys

Table 12.3 Requirements for GPS surveys

Survey type
Baseline length
(km)
#
satellites
GDOP
Observation
time
Accuracy
Static
20-50
≥ 4
≤6
2 - 3 hr
5 mm + 1 ppm
50-100
≥ 4
≤6
min. 3 hr
5 mm + 1 ppm
> 100
≥ 4
≤6
min. 4 hr
5 mm + 1 ppm
Rapid Static
0 - 5
≥ 4
≤5
5 - 10 min
5 - 10 mm + 1 ppm
5 - 10
≥ 4
≤5
10 - 15 min
5 - 10 mm + 1 ppm
10 - 20
≥ 4
≤5
10 - 30 min
5 - 10 mm + 1 ppm
Reoccupation
0 - 5
4
≤8
5 min
10 mm + 1 ppm
5 - 10
4
≤8
5 - 10 min
10 mm + 1 ppm
10 - 20
4
≤8
10 - 20min
10 mm + 1 ppm
 
It may be noted that 3 mm + 0.5 ppm accuracy is achievable using choke-ring antenna & long observation period in static mode. Table 12.4 provides some requirements for different GPS survey types:
 

Table 12.4 General requirements for GPS surveys (NAVSTAR Global Positioning System Surveying, 2003)

Method
Basic
Concepts
Obserrvation
Time
Accuracy
(3D rms)
Comments
Static
(PP)
  • L1 or L1/L2 receiver
  • simultaneous site occupation

30 min to 4 hr

(Rl 15 sec)

Sub-cm

(5 mm+ 1 ppm to 10 ppm)
  • Complexity varies
  • Control surveys (high accuracy,
  • Slow point positioning
Rapid Static
(PP)
  • L1/L2 receiver
  • sophisticated techniques and extra information to resolve ambiguities
  • Single occupation only
  • No continuous lock

3 to 5 min

(RI: 5 sec)

Sub-cm

(5 mm+ 1 ppm to 10 ppm)

  • Control surveys (medium to high accuracy) generally for baselines under 10 km, need "extra" measurements
Semi-Kinematic
(Stop & Go) (PP)
  • L1 receiver,
  • Needs initialization
  • 1-2 min baseline obsn. rover stopped temporarily on points to be positioned with respect to monitor
  • Must maintain continuous lock while moving between points
~1 min per point (2 epochs, RI: 1-5 sec)

cm+

(1-2 cm + 1 ppm)

  • Limited to baselines under ~10 km,
  • Control surveys (medium accuracy)
  • Fast point positioning
Pseudo-Kinematic (PP) Reoccupation
  • L1 receiver
  • Each rover site occupied twice, at least one hour apart, to exploit change in satellite geometry
  • 5-10 min static obsn
  • Double site occupation essential, logistically cumbersome
  • No initialization needed
  • Loss of lock permitted
3 - 5 min

cm+

(1 cm + 1 ppm)

  • Control surveys (medium accuracy)
Real Time Kinematic RTK
  • L1/L2 receiver
  • Data link needed
  • For baseline < 10 km
  • Moving rover positioned with respect to static monitor,
  • OTF initialization or conventional static initial ambiguity resolution
-
cm+
  • Logistically difficult since must maintain lock while moving
  • Location survey
  • Photo control
  • Real time topographic surveys
  • Construction stake out (medium to high accuracy)
SPP  
30 - 60 sec
10 - 100 m
 
SPP with PP  
5 to 10 hrs
3 - 5 mm
 
Kinematic & AROF  

1 sec

(RI: 0.2 sec)

1-2 cm + 1 ppm
  
Code difference with RT  
1 sec
0.3 - 0.5 m
 
Phase difference with RT  
1 sec
1-2 cm + 2 ppm
 
   
 
SPP: Single Point Positioning PP: Post Processing
RT: Real Time
RI: Recording Interval
AROF: Ambiguity Resolution on the Fly

 
In the above table, the recording/sampling or data interval refers to downloading interval of GPS data. It may be as high as between 0 - 1 second and as slow as 999 to 3600 seconds. High rate results in large volume of data and needs large storage. It also helps detection of cycle slips and results in improved accuracies for long baselines (> 50 km) when detection and repair of cycle slips is difficult.
 
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